The present invention relates to a method for reducing the frictional drag of a ship and to a ship with reduced frictional drag. In particular, the present invention relates to a technique by which energy required for delivering gas into water is reduced, and thereby power required for sailing a ship is effectively decreased.
This application is based on Japanese Patent Application No. 20000-067649, the contents of which are incorporated herein by reference.
A technique relating to a ship with reduced frictional drag is disclosed in Japanese Unexamined Patent Application, First Publication, No. 50-83992, Japanese Unexamined Patent Application, First Publication, No.53-136289, Japanese Unexamined Patent Application, First Publication, No. 60-139586, Japanese Unexamined Patent Application, First Publication, No. 61-71290, Japanese Utility Model Application No. 61-128185, and Japanese Utility Model Application No. 61-128185, etc. In a ship with reduced frictional drag, a number of microbubbles are produced so as to be present on a shell plating of a hull by delivering gas, such as air, into water from the outer surface of the hull (i.e., the shell plating) while the ship is in a sailing mode so that frictional drag generated between the hull and the water may be reduced by the presence of the microbubbles.
As a technique relating to the above-mentioned ship with reduced frictional drag, the applicant of the present invention proposed a technique by which microbubbles are produced to be present at a shell plating of a hull by delivering gas (for instance, air) into water from the vicinity of the bow. The aim of the technique is to cover the shell plating with microbubbles, which are generated by delivering gas from the vicinity of the bow, by diffusing the microbubbles along the flow of water on the shell plating. Conventionally, a gas supply device, such as a blower, has been used as a driving source for delivering the gas into water.
However, if a gas supply device, such as a blower, is used for delivering gas, a portion of the sailing power which is saved by the generation of microbubbles is lost because the power is consumed for driving the gas supply device. In the case where microbubbles are delivered at the vicinity of the ship""s bottom, in particular, a large driving force tends to be required for the delivery of gas since it is necessary to deliver the gas using a larger force than the static pressure present in the vicinity of the ship""s bottom. Also, a large cost for the device, operation, etc., will be required if a gas supply device is installed.
The present invention takes into consideration the above-mentioned circumstances with the following objects:
(1) To effectively decrease the power required for sailing a ship by reducing power necessary for delivering gas into water, and
(2) To reduce the construction cost of a ship.
In order to achieve the above objects, the present invention provides a method for reducing frictional drag between a hull and the water by generating bubbles in the vicinity of the surface of a shell plating of the hull in which a technique to discharge bubbles into water is adopted so that a negative pressure portion, whose pressure becomes lower with respect to a gas space as the ship travels, is formed in water, and gas is introduced into the negative pressure portion in water from the gas space.
Also, the present invention provides a ship with reduced frictional drag which reduces frictional drag between a hull and the water by generating bubbles in the vicinity of the surface of a shell plating of the hull, in which a technique to provide a negative pressure formation part which is disposed at the shell plating of the hull in order to form a negative pressure portion, whose pressure is lower than the pressure of a gas space, in water, and a gas path which introduces gas into the negative pressure portion in water from the gas space, is adopted.
Here, an explanation is made for reducing frictional drag of a hull by using the above-mentioned techniques. FIG. 1 is a schematic diagram showing a ship 10 with reduced frictional drag according to an embodiment of the present invention. In FIG. 1, the numeral 11 indicates a shell plating of a hull, the numeral 12 indicates a negative pressure formation part, the numeral 13 indicates a gas path, and the numeral 14 indicates a water level (i.e., the waterline). A flow of water 20 is created relative to the ship 10 as the ship 10 sails in the direction indicated by the arrow Xa in the figure at a predetermined speed Vh.
The ship 10 with reduced frictional drag forms a negative pressure portion 21 in water whose pressure becomes lower (negative pressure, vacuum) than the pressure of a gas space (atmospheric air) during the travel. That is, the flowing state of water is changed to a desirable state using the negative pressure formation part 12 which is disposed at the shell plating 11, in order to form the negative pressure portion 21 in water (a negative pressure system).
As a means for forming the negative pressure portion 21 in water, for instance, as shown in FIG. 2, the flow rate of water passing through a passage along the shell plating 11 of the hull may be increased by narrowing the passage by means of the negative pressure formation part 12 (Bemoulli""s theorem). In this case, the pressure P at the passage may be expressed by the following equation:
P=P0+xcfx81xc2x7gxc2x7hxe2x88x92xcfx81xc2x7(V12xe2x88x92Vh2)/2 xe2x80x83xe2x80x83(1) 
where V1 indicates the flow rate of water, P0 indicates the pressure of the gas space (atmospheric pressure), xcfx81 indicates the density of water, g indicates the gravitational acceleration, and h indicates the depth of water. As is apparent from equation (1), it is possible to form the negative pressure portion 21 in water by sufficiently increasing the flow rate of water V1 at a particular portion with respect to the speed of the ship Vh.
Also, it is known that a low-pressure portion (a flow separation area) tends to be generated behind an object placed in a flow because the matter changes the flowing state and causes a separation of a boundary layer of the fluid. That is, the negative pressure portion 21 may also be formed by generating a flow separation area in water by the above-mentioned negative pressure formation part 12.
In general, if a bluff body which increases the resistance against a flow is placed in a uniform flow, a flow separation area associated with irregular vortexes is generated in the downstream area immediately behind the bluff body. For example, if a cylinder is put in a uniform flow, a fluid flows along the cylinder at decreasing pressure until it reaches a minimum pressure point, and immediately after that, it separates from the surface of the cylinder and a flow separation area is formed. In this case, it is confirmed by experiment that the pressure at the minimum pressure point may be expressed as, for instance,
(Pxe2x88x92P0)/(xcfx81V2/2)≈xe2x88x922.2 xe2x80x83xe2x80x83(2) 
(where P is static pressure, P0 is reference pressure, xcfx81 is the density of fluid, and V is the flow rate). Accordingly, if the flow rate V is 7 m/s (about 14 knots), and the reference pressure P0 is 1 kgf/cm2 (atmospheric pressure), the static pressure P (absolute pressure) is calculated to be about 0.45 kgf/cm2, which is a negative pressure with respect to the atmospheric pressure. This indicates that it is possible, if the depth of water is about 5.5 m or shallower, to generate a flow separation area of negative pressure on the surface of a cylinder by flowing the cylinder in water at a speed V of 7 m/s.
According to the ship 10 with reduced frictional drag of an embodiment of the present invention, the negative pressure portion 21 is formed in water in the above-mentioned manner, and gas is supplied from a gas space at the high pressure side to the negative pressure portion 21 at the low pressure side in water via the gas path 13 in order to discharge bubbles 22 in water. In this manner, the shell plating 11 of the hull is covered by the bubbles 22 and the frictional drag between the ship 10 and the water is decreased.
Now, in a static liquid having a density xcfx81 as shown in FIG. 3A, in general, the energy E which is required for delivering a bubble having the volume Av (the density of the bubble is considered to be nil) to a position at the depth h from the liquid level may be expressed as:
E=(Pxe2x88x92P0)xc2x7Av xe2x80x83xe2x80x83(3) 
where p indicates the pressure (=xcfx81xc2x7gxc2x7h) at a delivering position of the gas. As is obvious from the equation (3), for the case where the pressure P at the delivering position of the gas becomes lower than the atmospheric pressure P0(P less than P0), the energy becomes negative (E less than 0), and hence, theoretically, the energy for delivering the gas becomes unnecessary.
That is, as shown in FIG. 3B, when gas is delivered into water by forming the negative pressure portion 21 in water using a technique according to the present invention, energy for delivering the gas to the delivering position of gas (at the depth h) is only required as a main power for delivering the gas. The energy is obtained by changing the flowing state of water using a negative pressure formation part, and is included in the driving force (navigation force) of the ship.
On the other hand, in a conventional method for reducing a frictional drag, gas is pressurized by using a gas supply device 30, such as a blower or a pump used as a pressurizing means, when delivering the gas into water as shown in FIG. 3C. In this case, in addition to the energy for delivering gas to a delivering position of gas (at the depth h), energy for pressurizing the gas by overcoming the hydraulic pressure P1 (i.e., the energy expressed by the equation (3)=energy for increasing the internal energy of the gas) is required as the power necessary for delivering the gas.
That is, in decreasing the frictional drag of a ship, the power required for delivering gas into water may be reduced by adopting a technique according to the present invention. When gas is delivered to a place of large depth, such as the bottom of a large ship, in particular, a great amount of energy is consumed if a conventional method is used because it is necessary to pressurize air to overcome the large static pressure (hydraulic pressure). However, according to the present invention, gas may be easily delivered into water by simply forming a negative pressure portion in water. Since it is considered that the shape of a negative pressure formation part or the Reynolds number is a main factor dominating in the formation of a negative pressure portion and that disadvantages due to the water depth do not tend to be caused in the formation of a negative pressure portion, the technique according to the present invention has advantages when applied to a large ship.
Also, the movement of bubbles generated in water differs in the above-mentioned negative pressure system according to the present invention and in a conventional pressurized system. The main cause of this is the difference in the pressure inside the bubbles immediately after being generated in water.
That is, in a conventional pressurized system, the size of the bubbles is not significantly changed when the bubbles move at a constant depth since the bubbles generated have an internal pressure which is substantially the same as the static pressure corresponding to the water depth.
On the other hand, according to the negative pressure system of the present invention, since bubbles generated have an internal pressure which is lower than the static pressure corresponding to the depth of water, the size of the bubbles is gradually decreased when they move at a constant depth (for instance, when the bubbles move along the bottom of a ship) separating away from the negative pressure portion due to the large hydraulic pressure applies onto the bubbles.
According to studies carried out by the applicant of the present invention, it is recognized that bubbles of relatively small size are more advantageous for decreasing the frictional drag of a ship. That is, the present invention has an advantage in that bubbles of small size, which are considered to be advantageous for reducing the frictional drag, may be easily generated by delivering gas using the negative pressure system. Also, since the bubbles tend to flow from a portion of high pressure to a portion of low pressure, a force is applied to the bubbles in the direction toward the negative pressure portion. Accordingly, the bubbles do not relatively tend to separate away from the shell plating of the hull, and it becomes possible to effectively use the bubbles for reducing the frictional drag. Note that the amount of bubbles generated at the negative pressure portion 21 is affected by the saturated vapor pressure which is determined based on environmental conditions in the vicinity thereof. That is, an amount of gas which is larger than that may be dissolved in flowing water is present as bubbles in water.